@article {MR3058191,
    AUTHOR = {Baumg{\"a}rtel, Hellmut},
     TITLE = {On a critical radiation density in the {F}riedmann equation},
  SJOURNAL = {J. Math. Phys.},
  JOURNAL = {Journal of Mathematical Physics},
    VOLUME = {53},
      YEAR = {2012},
    NUMBER = {12},
     PAGES = {122505, 9},
      ISSN = {0022-2488},
     CODEN = {JMAPAQ},
   MRCLASS = {83C10},
  MRNUMBER = {3058191},
       DOI = {10.1063/1.4771668},
       URL = {http://dx.doi.org/10.1063/1.4771668},
}